To achieve high-speed printing, an electrophotographic color image forming apparatus is known to include independent image forming units for respective colors, sequentially transfer images from the image forming units for respective colors onto an intermediate transfer belt, and collectively transfer images from the intermediate transfer belt onto a recording medium.
Each of the image forming units for respective colors includes a photosensitive drum as an image bearing member. Each image forming unit further includes a charging member for charging the photosensitive drum and a developing unit for developing a toner image on the photosensitive drum. The charging member of each image forming unit contacts the photosensitive drum with a predetermined pressure contact force to uniformly charge the surface of the photosensitive drum at a predetermined polarity and potential by using a charging voltage applied from a voltage power supply dedicated for charging (not illustrated).
The developing unit of each image forming unit applies toner to an electrostatic latent image formed on the photosensitive drum to develop a toner image (visible image).
In each image forming unit, a primary transfer roller (primary transfer member) facing the photosensitive drum via the intermediate transfer belt primarily transfers the developed toner image from the photosensitive drum onto the intermediate transfer belt. The primary transfer roller is connected to a voltage power supply dedicated for primary transfer.
A secondary transfer member secondarily transfers the primarily transferred toner image from the intermediate transfer belt onto a transfer material. A secondary transfer roller (secondary transfer member) is connected to a voltage power supply dedicated for secondary transfer.
Japanese Patent Application Laid-Open No. 2003-35986 discusses a configuration with which each of four primary transfer rollers is connected to each of four voltage power supplies dedicated for primary transfer. Japanese Patent Application Laid-Open No. 2001-125338 discusses control for changing, before image formation operation, a transfer voltage to be applied to each primary transfer roller depending on sheet-passing durability of an intermediate transfer belt and a primary transfer roller and on resistance variation due to environmental variation.
However, a conventionally known primary transfer voltage setting has the following problem. Since a suitable primary transfer voltage needs to be set in each image forming unit, a plurality of voltage power supplies is required. This increases the size of the image forming apparatus and the number of high-voltage power supplies, resulting in a cost increase. Since a suitable primary proper transfer voltage is calculated before image formation in consideration of resistance variation of the primary transfer member, it may take time until image formation is started.